In performing welding (seam welding) at the time of making a large diameter steel pipe, submerged arc welding with two or more electrodes is used. From a viewpoint of enhancing production efficiency of steel pipes, highly efficient welding work has been performed. That is, double one layer welding where an internal side is welded by 1 pass and an external side is welded by 1 pass has been carried out (patent documents 1, 2, for example).
In double one layer welding, an internal weld metal and an external weld metal overlap with each other and hence, it is necessary to secure a sufficient penetration depth (or depth of fusion) so as to eliminate an unmelted part, whereby welding is performed by applying a large-current of 1000 A or more in general. However, since an emphasis is placed on the enhancement of welding efficiency and the suppression of defects, a welding heat input becomes excessively high, thus giving rise to a tendency that the toughness of a weld, particularly, the toughness of a welded heat affected zone is deteriorated.
Although the reduction of welding heat input is effective to increase the toughness of a weld, unless a heat input in seam welding is largely reduced compared to a heat input usually carried out, a clear toughness enhancement effect cannot be acquired, and when a heat input is largely reduced, an amount of deposit metal is also reduced so that it is necessary to reduce a cross-sectional area of groove by an amount corresponding to the reduction of the deposit metal. Accordingly, the internal and external weld metals do not overlap with each other unless further deeper penetration welding is performed, thus increasing a possibility that a lack of penetration occurs.
Accordingly, to increase the toughness of the weld, both the large reduction of a heat input supply and the increase of a penetration depth are necessary however, the achievement of which is extremely difficult, although various proposals have conventionally been made.
For example, the above-mentioned patent document 2 proposes a submerged arc welding method where a penetration depth is increased by increasing current density corresponding to an electrode diameter. However, an electric current and current density are insufficient for the latest specification and hence, it is difficult to realize both the large reduction of a heat input and the increase of a penetration depth.
Patent document 3 proposes a submerged arc welding method with a high electric current and with a much higher current density. Here, an attempt is made to realize both the reduction of heat input and the deep penetration, by supplying an arc energy in the plate thickness direction as much as possible so as to ensure the depth of infusion by a required amount thus suppressing melting of a base material in the widthwise direction whereby an excessive welding heat input is eliminated.